Material for insulation system, insulation system, external corona shield and an electric machine

ABSTRACT

High-voltage insulation systems are simplified and can have a thinner design by the use of an electrically conductive PTFE fabric, wherein the thermal conductivity is also improved.

The invention relates to a material for insulation system, to aninsulation system, to an external corona shielding and to an electricalmachine.

Potential grading makes it possible in rotating machines, such asgenerators or high-voltage motors for example, to minimize electricalvoltages (potential differences), as a result of which the occurrence ofpartial and/or corona discharges can be reduced or entirely avoided.

In rotating electrical machines, the reliability of the insulatingsystem is decisively responsible for their operational reliability. Theinsulating system has the task of permanently insulating electricalconductors (wires, coils, bars) from one another and from the laminatedstator core or the surroundings. The external potential grading has thetask of establishing electrical contact between the laminated statorcore which is at ground potential and the outer side of the maininsulation. This ensures that no partial discharges occur in voids inthe region of the boundary layer of the insulation and the laminatedcore.

A distinction must be made here between

-   -   external corona shielding (ECS) for generator winding bars which        have been produced by single bar production (ECS-S) and    -   external corona shielding (ECS) for generator winding bars which        have been produced by means of the GVPI process (ECS-G).

In the case of globally impregnated stator windings (Global VacuumPressure Impregnation GVPI), the entire laminated core with a fullyfitted winding is impregnated and cured altogether. As a result, theadhesive bonding of the winding in the slots of the laminated core is sostrong that the different coefficients of expansion of the copper, ironand insulation lead to high thermomechanical stresses between thecopper, insulation and iron, which may lead to the boundary surfacestearing apart after a certain number of thermal cycles (starts and stopsof the generator). In order to prevent the gaps from being subject to adifference in electrical potential, and the partial discharges ignitingthere from destroying the insulation, an external potential grading(outer corona protection, OCP) is used, represented in FIG. 1 asdouble-layered external corona shielding, such as is used for thepotential grading of machines processed by means of G-VPI. An insulatingbase winding 70 of fine-mica tape is applied over the current-carryingRoebel bar of copper conductor elements 40, said insulating base windingsmoothing and increasing the edge radii of the thin copper conductorelements 40. A first graphite-containing conductive nonwoven tape 100 iswound over said insulating base winding, said conductive nonwoven tapebeing connected to the high-voltage potential of the copper conductorelement 40 by way of a contact strip 130 at only one point.

Only then is the main insulation 160 of fine-mica glass wound. Insteadof the copper conductor components, the first conductive nonwoven tape100 now forms the high-voltage electrode. It is permanently adhesivelybonded to the main insulation.

Following on top of the main insulation 160 is the inner external coronashielding winding 110 which is composed of a material according to theprior art (conductive, flexible tape, in particular from Krempel), anoutermost separating tape 190′ and an outer external corona shieldingwinding 200. An external corona shielding tape 140, which is woven inthe outermost separating tape 190′, connects the inner external coronashielding winding 110 and the outer external corona shielding winding200.

The thermomechanical stresses occurring between the copper conductorassembly and the insulation during the starting and stopping of thegenerator may after a certain operating time lead to instances of localdetachment of the insulating sleeve from the conductor, without thefeared partial discharges igniting in the gaps that are produced. Theregion of the delamination is potential-free, because the high-voltagepotential has been transferred to the conductive nonwoven that becomesbaked fast on the main insulation. This IPG design at the highlystressed inner boundary layer between the conductor and the insulationallows turbogenerators to be operated at peak load for decades withoutany notable aging due to partial discharge.

The object of the invention is therefore to solve the abovementionedproblems.

The object is achieved by an insulation material as claimed in claim 1,an insulation system as claimed in claim 17, an external coronashielding as claimed in claim 20 and an electrical machine as claimed inclaim 21.

Further advantageous measures that can be combined as desired with oneanother in order to achieve further advantages are listed in thedependent claims.

In the drawings:

FIG. 1 shows an outer potential grading of a generator winding baraccording to the prior art,

FIG. 2 shows an outer potential grading according to the invention, and

FIG. 3 shows a generator.

The figures and the description represent only exemplary embodiments ofthe invention.

The invention involves using hydrophobic material, in particular PTFE(Teflon) as insulation, in particular for a high-voltage insulationsystem of this kind, wherein the insulation system is designed to beelectrically conductive in the form of layers.

The high-voltage insulation system may be a simple system or a morecomplex system as in FIG. 1.

The invention is explained only on the basis of PTFE as an example of ahydrophobic material.

The hydrophobic material or PTFE is preferably already designed to beelectrically conductive. The PTFE is then a composite material.

This preferably takes place during the production of the material, inparticular by means of mixing in electrically conductive material, inparticular graphite, for example by means of extrusion duringproduction, with fibers then preferably being produced.

However, subsequent electrically conductive coating of a woven fabric, afiber, a laid scrim, a diaphragm or a film is also possible.

The woven fabric is preferably formed from fibers which comprise theelectrically conductive insulation material, in particular PTFE.

This woven fabric is preferably present in tape form and for theapplication is wound onto the surface to be insulated (see FIG. 2).

A perforated tape (in woven fabric form or the like), a perforateddiaphragm or a perforated laid scrim (laid scrim as is known fromtextile technology) can likewise be used, that is to say through-holesare made in the tape, in the woven fabric, in the diaphragm (which isalready porous) or in the laid scrim.

The high-voltage insulation system therefore preferably comprises, inthe external corona shielding, fibers or a woven fabric composed ofPTFE, wherein there is preferably also an electrically conductivematerial, preferably graphite, between the woven fabric-formingstructures to achieve the electrical conductivity.

The ECS in the high-voltage insulation system in FIG. 2 preferablycomprises a woven fabric composed of PTFE, wherein this is structurallydesigned such that the woven fabric has pores which can be infiltratedas above in accordance with the described process.

The basic design and manner of operation of the current ECS systemaccording to FIG. 1 is intended to remain unchanged in the process, withthe exception of the omission of the external corona shielding tape 140and the replacement of the mica splittings with the PTFE-containingmaterial according to the invention. The external corona shieldingwinding 200 in FIG. 2 can also be dispensed with.

This yields the following advantages:

-   -   good impregnability, since it is a porous woven fabric and can        therefore be applied before the curing.    -   unchanged resistance before and after the impregnation, since        the conductivity is attributable to fibers and not to particles        as in the case of ECS tape. (These have a different resistance        value in comparison to the initial value on account of the        polymer matrix enveloping the particles after the impregnation).

The objectives for the ECS-G according to the invention are:

-   -   simplified application/cost reduction    -   reduced layer thickness of the double ECS by thinner        alternatives.

The approach for the ECS-G according to the invention is:

-   -   reduction of the layer thickness by using a separating layer,        which provides defined mechanical decoupling without causing the        electrical resistance to change. This is intended to be        accomplished by replacing the double layer of mica splittings        with hydrophobic types of woven fabric. This may be, in        particular, a Teflon material. The structure is made up in the        following way:

An improvement is obtained according to the invention by the use ofelectrically conductive woven fabric 190 composed of PTFE, since thismakes the “interweaving” of the external corona shielding tape 140(FIG. 1) unnecessary. This would make it possible to reduce the layerthickness and the production complexity (FIG. 2).

The structure according to the invention of an innovative outerpotential grading for use in the GVPI process allows an insulationsystem that corresponds to the current state of the art in respect ofproperties but has the benefits of:

-   -   establishing freedom from partial discharges after curing    -   comparable loss factors after carrying out thermal cycling tests        for accelerated thermomechanical loading    -   comparable long-term electrical stabilities under operational        voltage loading and with increased voltage loading    -   comparable long-term electrical stabilities under operational        voltage loading and with increased voltage loading after        artificial aging in different thermal cycles.

These investigations were carried out on generator winding bars with thefollowing design:

-   -   aluminum profiles with a length of approximately 1.5 m and        dimensions of 1 cm×5 cm    -   number of layers of mica 8+1 layer of IPC for a rated voltage of        13.8 kV    -   number of generator winding bars per collective: 6.

In this case, a reduction of the layer thickness of the current ECS ofapproximately 450 μm to a value of about 100 μm was made possible.

FIG. 3 shows, by way of example, a generator as the electrical machine.

According to FIG. 3, a rotary machine arrangement, in particular agenerator arrangement 2, extends along a longitudinal axis 3 from aturbine-side end region 6 to an excitation-side end region 8. Thegenerator arrangement 2 has a housing 11. A cooling device 12 isarranged in the turbine-side end region 6. To be precise, two coolers 16and a compressor in the form of a fan 18 having a fan hub 20 arearranged in a cooler head 14 which is a part of the housing 11. The fanhub 20 is seated on a rotor 22 which extends along the longitudinal axis3 through the generator arrangement 2. The actual generator region 23 isarranged so as to follow the cooling device 12 in the direction of thelongitudinal axis 3. In this region, the rotor 22 is surrounded by astator 24 such that an air gap 26 is formed. The stator 24 has a statorwinding having a turbine-side stator winding overhang 28A and having anexcitation-side stator winding overhang 28B. A so-called laminated core30 is arranged between the two stator winding overhangs 28A, 28B.Analogously to the stator 24, the rotor 22 has a turbine-side rotorwinding overhang 32A and an excitation-side rotor winding overhang 32B.

On account of the high power density that is customary inturbogenerators, it is necessary to cool the generator arrangement 2 inthe generator region 23. In this case, the stator winding overhangs 28A,28B and also the rotor winding overhangs 32A, 32B have a particularlyhigh cooling requirement. In order to cool the generator region 23, saidgenerator region has a cooling system 34 which is supplied with coolinggas by the cooling device 12. The cooling system 34 has a number ofcooling gas ducts 36A, D, 48 via which the cooling gas is circulated. Inthis case, a first cooling gas duct 36A extends in the axial directionand is arranged between the stator 24 and the housing 10. A secondcooling gas duct 36B is formed by the air gap 26. Further cooling gasducts 36C which extend in the axial direction lead through the laminatedcore 30. In order to cool the rotor 22, a cooling gas duct 36D leadsthrough said rotor. The cooling gas flow in the generator region 23 andalso in the cooling device 12 is indicated in each case by arrows,wherein the dashed arrows indicate the flow path of the cold cooling gasand the solid arrows indicate the flow path of the heated cooling gas(hot gas).

In order to cool the stator winding overhangs 28A, 28B, the cooling gasflow coming from the coolers 16 is divided in the turbine-side endregion 6. One partial flow serves for cooling the turbine-side statorwinding overhang 28A and the other partial flow is forwarded via thecooling gas duct 36A to the excitation-side stator winding overhang 28Band divided once again. One part serves for cooling the stator windingoverhang 28B and flows back again from there as hot gas via the air gap26. The other part is conducted through the cooling gas ducts 36C of thelaminated core 30 and emerges as hot gas in the turbine-side end region6 and is fed to the coolers 16. In order to cool the rotor windingoverhangs 32A, 32B, cooling gas is introduced into the cooling gas duct36D of the rotor 22 both from the turbine-side end region 6 and from theexcitation-side end region 8. A partial flow of the cooling gas flowsthrough the respective rotor winding overhangs 32A, 32B and issubsequently conducted into the air gap 26 as hot gas and fed to thecoolers 16. The remaining partial flow is guided further through therotor 22 in the cooling gas duct 36D, to be precise in such a way thatthe cooling gas from the two rotor winding overhangs 32A, 32B flowstoward one another and is conducted into the air gap 26 approximately inthe central region 38 of the generator region 23.

1. An electrically conductive insulation material, which compriseshydrophobic material comprised of fibers, and the fibers are comprisedof hydrophobic, electrically conductive material.
 2. The insulationmaterial as claimed in claim 1, wherein the electrically conductive,hydrophobic material comprises electrically conductive PTFE.
 3. Theinsulation material as claimed in claim 1, which is comprised of a wovenfabric composed of fibers, which at least partially comprises the fibersof hydrophobic, electronically conductive material. 4-5. (canceled) 6.The insulation material as claimed in claim 3, wherein the hydrophobic,electrically conductive material of the woven fabric, is composed ofelectrically conductive PTFE.
 7. The insulation material as claimed inclaim 1, configured in the form of a tape.
 8. The insulation material asclaimed in claim 1, wherein the material is perforated and configured tobe infiltrated. 9-10. (canceled)
 11. The insulation material as claimedin claim 1, comprised of a perforated tape composed of electricallyconductive, hydrophobic material. 12-13. (canceled)
 14. The insulationmaterial as claimed in claim 1, wherein the hydrophobic material is inthe form of one of the fibers, a woven fabric, a diaphragm, a laid scrimor a tape, and each coated with an electrically conductive layer. 15.The insulation material as claimed in 1, wherein the electricallyconductive material comprises graphite. 16-19. (canceled)
 20. Anexternal corona shielding, comprising an insulation system including aninner external corona shielding winding and an outer external coronashielding material and PTFE containing material according to claim 2arranged between the inner external corona shielding winding and theouter external corona shielding material as claimed in one or more ofthe preceding claims, in particular consisting of said insulationmaterial or an insulation system.
 21. (canceled)
 22. The insulationmaterial as claimed in claim 1, configured in the form of a diaphragm.23. The insulation material as claimed in claim 1, which configured inthe form of a laid scrim.
 24. The insulation material as claimed inclaim 1, comprised of an electrically conductive material mixed into ahydrophobic material.
 25. The insulation material as claimed in claim 3,wherein the electrically conductive material is distributed between thefibers or in the woven fabric structure.
 26. The insulation system asclaimed in claim 13, which has pores configured to be infiltrated inparticular.
 27. An electrically conductive insulation system whichcomprises a hydrophobic material.